FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for fabricating semiconductor single crystals by using the Czochralski Method (the CZ method).
At present, most semiconductor substrates used for fabricating semiconductor components are single crystals of silicon with high purity. FIG. 8 is a cross-sectional diagram showing a semiconductor single crystal fabricating apparatus provided with a shield cylinder surrounding the semiconductor single crystal being lifted. As shown in FIG. 8, within the main chamber 1, a graphite crucible 18 is disposed upon the upper end of a rotary crucible shaft 17 which is able to be driven to extend upward or downward. A cylindrical heater 16 and a keep-warm cylinder 19 are disposed around the crucible 18.
Polycrystalline silicon in lumps is put into a quartz crucible 14 which is accommodated within the graphite crucible 18, then the polycrystalline silicon is heated by the heater 16 to be melted into a melt 20. A seed crystal in a seed holder 21 is immersed into the melt 20, and thereafter the seed holder 21 is slowly withdrawn and rotated in a direction the same as or counter to that of the rotation of the graphite crucible 18 to grow a single crystal silicon 7.
A graphite shield cylinder 22 is suspended and extended to above the melt 20 within an upper chamber 2 which is connected to the main chamber 1. The graphite shield cylinder 22 is engaged with an ascent and descent mechanism (not shown) so as to perform an upward or a downward movement when intended. The graphite shield cylinder 22 controls the flow of inert gas coming from a source above the upper chamber 2 and obstructs heat radiation coming from heater 16 and melt 20. By this arrangement, the single crystal silicon 7 being lifted can be cooled or kept warm throughout the whole temperature zone, thereby expediting the crystallization and accordingly enhancing the productivity of the single crystal silicon 7.
The heat radiation coming from the parts within a hot zone (for example, the heater 16) toward the single crystal silicon 7 being lifted is obstructed by the graphite shield cylinder 22, thus the temperature gradients both in radial and axial directions near the solid/liquid boundary of the single crystal silicon 7 become large, and this leads to a easy crystallization of the single crystal silicon 7. In view of the above, it is possible to accelerate the lifting speed of the single crystal silicon 7, and the productivity can thus be enhanced. However, it is impossible to alter the thickness of the shield cylinder 22 in response to the surrounding circumstances within the heating furnace, nor is it possible to adjust the execution of cooling or heat obstruction at a designated portion of the single crystal silicon 7 being lifted. Therefore, the following disadvantages will happen:
(a) When the single crystal silicon 7 passes through the zone whose temperature is within a range between 1000.degree. C. and 1200.degree. C., it can not be cooled slowly. As a result, the as-grown defect density can not be reduced sufficiently. This will reduce the oxidation-film breakdown strength.
(b) In the operation of melting polycrystalline silicon in the quartz crucible 14, an ascent and descent mechanism is used to lift the upper portion of the shield cylinder 22 so as to accommodate it within the upper chamber 2. By this, interference between the lower end of the shield cylinder 22 and the polycrystalline silicon can be avoided. For this purpose, an accommodation space is required in the upper chamber 2, and the total height of the upper chamber 2 is thus increased.